Polymers of ethylene, for example, homopolymers of ethylene and copolymers of ethylene and higher alpha-olefins, are used in large volumes for a wide variety of end-uses, for example, in the form of film, fibers, molded or thermoformed articles, pipe, pipe coating and the like. There are two types of processes for the manufacture of polyethylene that involve the polymerization of monomers in an inert liquid medium in the presence of a coordination catalyst viz. those which operate at temperatures below the melting or solubilization temperature of the polymer and those which operate at temperatures above the melting or solubilization temperature of the polymer. The latter are referred to as “solution” processes, an example of which is described in Canadian Patent 660,869 of A. W. Anderson, E. L. Fallwell and J. M. Bruce, which issued Apr. 9, 1963. In a solution process, the process is operated so that both the monomers and polymer are soluble in the reaction medium.
U.S. Pat. No. 5,589,555 issued Dec. 31, 1996 to Zboril et al. describes a process for controlling a solution polymerization process by controlling the composition of the aluminum alkoxy alkyl used in activating the catalyst. The catalyst is prepared reacting a mixture of a dialkyl magnesium compound and a first aluminum compound with a reactive halide and titanium tetrachloride and optionally a vanadium oxytrihalide to make a first component. The resulting mixture is then reacted with a mixture of a second aluminum alkyl compound. The patent does not teach or suggest heating of the reaction product of the first three components to elevated temperatures from 120° C. to 180° C. before adding a second aluminum component.
The specification teaches at column 2 lines 54 through 64 that, optionally the first component may be prepared by reacting (i) the first aluminum alkyl compound, and (ii) the titanium tetrachloride containing optionally vanadium oxytrihalide at temperature of less than 30° C. and heating the resultant admixture to at temperature of 150-300° C. for a period of 5 seconds to 60 minutes before adding (iii) second aluminum alkyl. No magnesium compound is used in this option.
U.S. Pat. No. 5,492,876 issued Feb. 20, 1996 to Gillis et al. teaches a process to prepare a catalyst comprising mixing in an inert organic solvent, aluminum compound of the formula AlRnX3-n where R is a C1-20 alkyl, cycloalkyl and aryl radical, X is a halogen and n is 1, 2 or 3; with titanium tetrahalide and vanadium oxytrihalide and heating the resulting solution to a temperature from 180° C. to 250° C. for a time from 15 seconds to 5 minutes and cooling the resulting solution to a temperature of less than 100° C. and adding second vanadium oxytrihalide compound to the cooled mixture and then activating the mixture with an additional aluminum compound, preferably an aluminoxane. The present invention does not contemplate the addition of a vanadium oxytrihalide nor does it contemplate the cooling step. Additionally the reference does not teach the presence of a magnesium compound.
Typically in the solution polymerization of ethylene catalyst tends to produce lower molecular weight polymer (which is indicated by a higher melt index) and lose activity at high temperatures. Also in producing ethylene copolymers it is desirable to have more and more uniformly distributed co-monomer incorporation which is reflected by a lower density. It is desirable to find a catalyst having a high activity without significantly reducing the molecular weight of the polymer. The present invention seeks to provide such a catalyst.